When combustible gases of different origin are mixed, it is of great technical and economic importance to be able to determine the calorific value and/or the Wobbe index of the mixture quickly, simply and cheaply.
The Wobbe index is defined by the expression ##EQU1## where
H=the gross calorific value of the gas, in MJ per m.sup.3
d=the relative density of the gas with respect to air.
The Wobbe index is an important quantity in combustion engineering; when, at one and the same pressure, fuel gases of different composition but with the same Wobbe index are fed to gas-fired equipment, the heat produced is the same. When e.g. in an industrial plant a mixture of gases of different origin is used as fuel, stable operation requires that the gases be mixed in such a ratio, possibly with admixing of inert gas (in this connection, air may also be considered an inert gas), that a gas with a virtually constant Wobbe index is obtained.
From NL-A-7808476 it is known that when gases of different composition and different Wobbe index are burnt with equal quantities of air, the oxygen content of the flue gases can be directly correlated to the Wobbe index. This means that for measurement and control purposes it is not necessary to measure the Wobbe index as such, but it suffices to measure the oxygen content of said flue gases.
With the known method the Wobbe index or calorific value of a gas mixture can be determined within 30-60 seconds. However, in certain situations a quicker response of the measurement may be desirable. With the known (combustion) measuring method, this is impossible.
The object of the invention is, among other things, to provide a method of determining the gross calorific value of a combustible medium in which within a very short time, e.g. not more than 30 seconds, a virtually complete response of the measurement is obtained.